Method and apparatus for increasing the ionization of impurity ions in a mass spectrometer



May 31, 1966' R w. L. FITE ETAL 3,254,209

METHOD AND APPARATUS FOR INCREASING THE IONIZATION OF IMPURITY IONS IN A MASS SPECTROMETER Filed Nov. 29. 1962 2 Sheets-Sheet 1 W59 ".Z Z4 55g CATRODE f PULSE u FOLLOWER GENERATOR n j 52 cm1-wus RAY J0 osmLLoscoPE AMPUHER ,j fl@ \f\ mTEeRn-rme PEAK- `L` EF'. M VNV-W v.T.\/. M. PULSER I y 6 J4 Z0 J6 pans vormse SUPPLY wl-dll T -n j 1 `3Z 36* liv-TTI Jl) May 31, 1966 Filed NOV. 29, 1962 W L. FITE ETAL OF IMPURITY IONS IN`A MASS SPECTROMETER 2 Sheets-Sheet 1 A Y Z V 'zoo ...l 4 z xn- \oo N+ o No* 0 5o loo \so zoo TIME (/u sec) 5 45 pLsE PULSE //J g g "eenemon @msnm-ora 50 50 W @j 35 sa'. z'=. JZ LINEAR f PQLSE. ACCELEEATOE. Z C-ll j 3Z`.; H11-: 32N l kf/ 33 JZ lj) JZ 2.

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Z9/ 52 l' l /4 15k PULSE 3g 34 GENERATOR Z9 @ATE DELAY MWQY': Z4 JZ; 70 WQ// 7g VTV M 55 Wil/a//QJ/a United States Patent O 3,254,209 lVIETI-IOD AND APPARATUS FOR INCREASING THE IONIZATION OF IMPURITY IONS 1N A MASS SPECTROMETER 'Wade L. Fite, Encinitas, Calif., William R. Snow, Seattle,

This invention relates to mass spectrometers used for detecting very small impurities in various gases. More particularly, it relates to a mass spectrometer having an ion source in which impurity ions are produced in the afterglow of a pulsed gas discharge.

Mass spectrometers of the prior art comprise an ion source, an ion analyzing section and an ion detector. Gas is admitted to the ion source where it is ionized usually by impact of electrons from an electron gun. The ions are then accelerated, usually by an electric field, focused and passed into the anlyzing'section. The .analyzing sections most commonly in use achieve ion separation :by

passage of the ions through a magnetic iield, radio-frequency electric fields or by time of transit over a field-free distance. Separation of ions of different massesl is accomplished in the rst two cases fby the ion trajectories differing between ions (spatial separation) while the lastnamed method is a temporal separation process. The detector may comprise an electron multiplier which amplifies the signal for further utilization. These mass spectreme-ters of the prior art have sufficient sensitivity to permit the measurement of impurities present in the amount of parts per million parts of gas.

In mass spectrometers of most common usage, the ionization is produced by impact of electrons from an electron gun, containing 4a thermionic source of the electrons. Because the thermionic source is hot, and will react chemically with most gases causing deterioration of the electron source, considerations of operating lifetime of the electron gun dictate the use of low pressures of the -gases admitted to the ion source. The desirability of having the electrons from the gun being able to traverse the region contained by the source also recommends the use of low g'as pressures in the source. In practice, conventional mass spectrometers use gas pressures of the order of 10-3 mm. Hg and less. Under these circumstances, ions produced in the source can leave the source without undergoing appreciable additional collisions with the gas, so that the ions entering the spectrometer are primary ions. The primary ions are characteristic of the various gases in the source, and the ion currents produced are closely proportional to the partial pressures of the neutral gases that are in the source.

The sensitivity of a conventional mass spectrometer lfor the detection of impurities present in a Igas predominantly of one type depends primary on the range of sensitivity available at the ion detector. If one seeks to determine an impurity concentration of, say, 1 part per million in a gas, then it is necessary to have a range of detector sensitivityl of 1 part per million also, since the relative numbers of ions will be closely similar to the rela-tive numbers of the neutral molecules in the gas. Such a range of detector sensitivity is common in the practice of mass spectrometry. Appreciably greater range in detector sensitivity, and -therefore-over-all instrument sensitivity 4for detection of impurities, is not common, however; practical considerations such as noise levels in detector electronics, imperfect mass resolution between neighboring masses, etc., makes an increased range of sensitivity difticult to achieve.

The instant invention may be used when the ionization potential of .the impurity is less than the ionization po- 3,254,209 Patented May 31, 1966 ICC tential of the main part of the gas. That is, an impurity particle must be ionizable with less energythan the energy required to ionize a particle of the main part of the gas; Under these circumstances, any ions f the main gas that are produced, transfer their charge to neutral particles of the impurity, thus producing ions associated with the impurities (i.e. impurity ions or fragments thereof).' The rate of transfer depends upon the cross sections or reaction rates for the transfer processes, the relative ionization potentials, the number of ions of the main gas present, and the number of neutral particles of the impurity available to be ionized.

In accordance with the instan-t invention, ion source pressures are made higher than in conventional mass spectrometer ion sources so that ions produced therein undergo appreciable additional collisions with the gas whereby the primary ions can react with the gas to form secondary ions, through ion-atom interchange processes, charge transfer and possibly other heavy particle collision processes. These ion-neutral collision processes produce secondary ions at the expense of primary ions and in some cases can ybring about populations of ions which are Widely different from the populations of the neutral species in thesource gases. In many cases also, the number of secondary ions of a given chemical species will be much greater than the number of primary ions of -that species, yfor the ions of the species will have been created not only by primary electron impact ionization but tby subsequently heavy particle collision processes with primary ions of a diHerent species. As a result, in

' many cases the relative number of ions of a given species of impurity in a main gas can be selectively ampliiied in intensity bythe heavy particle collisions, and a given range of overall mass spectrometer sensitivity can be made to exceed Ithe range of detector sensitivity. This higher pressure is preferably of the order of 1 mm. Hg.

To simplify the explanation of the invention the simplest case will be explained first. In this case there is but one principal gas and one principal impurity so that competing processes need not -be taken into consideration. This is a real case, as it is frequently found that a gas contains only a single impurity `of any consequence.

One form of the instant invention requires a pulsed ion source, that is gas is ionized in the ion source in pulses, thereby producing pulses of ions of both the main'gas and the impur-ity. Means involving other than an electron Vgun can :be employed to produce ionization. A pulse may be made very short so that relatively small fractions of the main gas and the impurity inl the ion source are ionized. Following the termination of the ionizing pulse there is residual ionization of the gas until the ions recombine with electrons. This residual ionization is termed lthe afterglow for the reason that the ionized gas gives oft light during the recombination.

During the afterglow, if the gas is ionized to the extent of, say, one part per million, and the amount of impurity is much less then one part per million, the impurity particles will become ionized at the expense of the ions of the main gas by the transfer of charge from the ions of the main gas. These impurity ions may then be focused into a beam, separated magnetically or by other means, and then detected selectively, as in a conventional mass spectrometer. Under these conditions the number of ions -of the main gas will be less than in the conventional mass spectrometer whereas the relative number of impurity ions will `be increased bya large factor. In some favorable cases virtually all of the impurity particles be' of ions of the main gas present to furnish the charge during the afterglow. During the initial portion of the period of `charge transfer, and before the number of neutral particles of the impurity is depleted substantially, the rate of production of the impurity ions is substantially proportional to the product of the number of neutral impurity particles initially present, the number of ions of the main gas initially present, and the cross sections for the processes. By carefully cont-rolling the ionizing pulse, the number of ions of the main gas initially present can be made constant for each ionizing pulse. The ionization current for the impurity ion is measured by the detector of the mass spectrometer. T'he initial rate of increase of this current after an ionizing pulse is, under the aforesaid circumstances, proportional to the amount of impurity present. It can be measured in a conventional way, as by applying the detection signal to a synchronized oscilloscope, and measuring the slope of the resultant trace. The apparatus can be calibrated empirically by introducing a known amount of impurity into the main gas, and observing the slope of the resultant trace.

Alternatively one may measure the total ion flow of the impurity ion integrated over-all time following a given pulse. This will provide direct measure of the number of neutral particles of the impurities initially present. The relative presence of the impurity can also be measured by measuring the peak ion current of the impurity ion, which is also directly related to the amount of impu-rity initially present.

An alternative manner of measuring the slope of the impurity ion current is to measure the current at two predetermined times during the cycle, .and measuring the ratio of the two measurements. Since at the termination of the ionizing pulse, the impurity ion current is relatively negligible, a close approximation may be made by making but a single measurement of the current at a single predetermined time.

In each of the above alterative cases, the apparatus can be calibrated empirically by introducing a known amount of the impurity into the main gas and observing the desired signal.

When there is but a single significant impurity present in the main gas, then measurement is relatively simple. When there are 'other impurities present to compete for the transfer charge from the ions of the main gas, the rate of growth of ions of the respective impurities becomes more complex. Even when there are but two gases present, various multiatomic particles may compete for the charge. For example, if oxygen is present in nitrogen,

the nitrogen maybe ionized monatomically and diatomically, and charge is not only transferred between these ions and to monatomic and diatomic oxygen but also to particles comprising an atom of oxygen and an atom of nitrogen and from one to the other of all the Various particles. Other molecular ions may also -be produced. It has, therefore, been found preferable to measure the current yusing Oscilloscopes, for then all of the information is available, the initial slope of the ion current, the shape of t-he curve and the peak ion current, as well as the average ion current for each of the competing processes. The instrument may be calibrated empirically, releasing known main gases and impurities in known amounts.

Therefore, the principal object of this invention is to provide a new method and apparatus for measur-ing very small impurities in certain gases. A further object is to provide an improved mass spectrometer for measuring very small impurities in certain gases where the impurities are preferentially ionized. A still further object of the invention is to provide an improved method and apparatus for measuring very small impurities in certain gases by transferring charge from ions of the main gas to neutral particles of the impurities during the afterglow of a gaseous discharge in the gas. Another object of the invention is to provide an improved ion source for a mass spectrometer in which the ionization is pulsed and charge charge in the ion source provides large number of impurity ions;

FIGURE 2 shows typical -oscilloscope traces obtained for helium containing 50 parts per million of neon using the appara-tus of FIGURE l;

FIGURE 3 shows typical oscilloscope traces obtained for nitrogen containing oxygen using the apparatus of FIGURE l;

FIGURE 4 shows a modified form of ion source that can be used in the apparatus of FIGURE 1;

FIGURE 5 shows another modified form of ion source in which pulses of very -high speed electrons from an external electron accelerator are used to ionize the gas; and

.FIGURE 6 shows a modified form of read-out device that can be used in the apparatus of FIGURE l.

In FIGURE 1 there is illustrated one form of the invention in which a mass spectrometer is -used with a pulsed radio-frequency ion source and a synchronized oscilloscope. The apparatus includes a mass spectrometer in which the parts extending from mass spectrometer entrance slit 12 thorugh electron multiplier 14 are found in the prior art. Ions to be measured pass into the mass spectrometer through the slit 12, which provides a point source of ions. These :ions are focused into a beam in a conventional manner by electrical lens elements 16, to which appropriate voltages are supplied from lens voltage supply 18. The ions are formed into a beam and propelled into a magnetic field produced in the region 19 by an electro-magnet 20. This magnetic field causes the ions to move in circular paths. The radius of curvature of the paths depends upon the velocity and -mass of the ions of the beams. The velocity of the various ions also depends upon their mass inasmuch as all ions fall through the same electrical potential as -they pass through the lens elements 16. By appropriate selection of the current in the coils of the electromagnet 20 in a well-known manner, ions of particular mass are caused to leave the region 19 in a particular direction, whereas ions of other mass are caused to deviate by a greater or lesser amount from their original direction. Those moving in just the right direction pass through a mass spectrometer exit slit 22, and enter the electron multiplier 14. As shown in FIGURE 1, only those ions that have changed direction by pass through the exit slit 22. Of course, the exit slit could have been placed at any desired angle and the current through the coils of the electromagnet adjusted appropriately so that the ions of the desired mass change direction by just the right amount to pass through the exit slit to the electron multiplier. The electron multiplier is supplied with voltage in a conventional manner so that it may act in its usual fashion to amplify the applied signal. The ion current entering the electron multiplier through the exit slit 22 becomes, by electron multiplication, an amplified signal on output lead 24 which signal is directly proportional to the ion current passing through exit slit 22.

The interior of the mass spectrometer is constantly evacuated through tubes 26 and 28 by conventional means such as diffusion pumps.

The apparatus includes a particular ion source and a particular means for reading out the information from the signal on output lead 24. One form of ion source is illustrated in FIGURE l. As shown in FIGURE l the ion source 29 comprises a chamber 30 formed by chamber walls 32. Gas to be analyzed is supplied through a gas input tube 34 to the chamber 30. The chamber contains an electrode 36, called a pusher electrode, to which a D.C. voltage is applied by a battery 38 through a resistor 40. This voltage supplies a drift voltage which serves t-o push ions for-med in chamber 30 away from electrode 36. Some of these will pass through' the entrance slit 12 for analysis. Ions are produced 'in chamber 30 by the action of a radio-frequency eld produced between electrode 36 and the walls 32 by a signal from radio-frequency pulser 42 through condensers .44 and 46. The radio-frequency pulser may be of the sort that is disclosed in Electrical Design News, Rogers Publishing Co., Englewood, Colorado, September, 1958, p. 16. The pulser 42 produces bursts of radio-frequency energy upon receipt of pulses from a pulse generator 48. The pulse generator may be a standard pulse generator, such as Rutherford Model B-7. The pulse generator generates a pulse which can be varied in Width, repetition rate and amplitude. If necessary, an amplifier 50 may be used to amplify the signal from pulse generator 4S to control the operation of the radio-frequency pulser 42. Typically the radio-frequency pulser, 'under the control of the pulse generator, may emit bursts of 30 megacycle alternating current of duration about 5 microseconds and at a repetition rate of 1000 cycles per second. Each pulse applied between electrode 36 and source 32 ionizes gas contained in chamber 30; this is a gas discharge. The pulse is so short that the energy introduced into the chamber ionizes only of the order of lone part per million of the gas. Where the gas consists of impurities contained in amounts much less than one part per million of a lmain gas, and where -the ionization potential of the main gas is greater than the ionization potential of the impurity, charge is transferred from the ions of the main gas -to neutral particles of the impurity after the ionizing signal from the radiofrequency pulser l42 is terminated. This is the afterglow period of the discharge. Ata pressure of about 2 mm.

Hg, Vthe number of impurity ions will increase about 100% in a microsecond. As stated above, this increase will be linear over a certain period and, vother things being constant, will increase at a rate proportional to the relative amount of impurity in the gas. By the action of the drift field and the evacuation of the interior of the vmass spectrometer, the ions produced inthe chamber 30 pass through the entrance slit 12 and are analyzed according to their mass. Ions entering through entrance slit 12 are analyzed in a well known manner; and only ions of the mass of the impurity being measured, are permitted to pass through exit slit 12 to -be detected by the electron multiplier 14. The ion current of the desired ions is amplified and applied through a cathode follower circuit 52 to the deflection plates of a cathode ray oscilloscope. The cathode follower serves to match impedances. The cathode ray oscilloscope is also conventional. In the form of the apparatus illustrated in FIGURE l the cathode ray voscilloscope operates with a conventional saw-tooth sweep triggered in a conventional manner by a pulse from pulse generator 48 so that the trace on the cathode ray oscilloscope is synchronized with the production of ions in the chamber 30. Typically the y ion current of the impurity ion .builds up for a period of the order of 100 microseconds. the signal of the order of 100 times and improves the overall sensitivityv of the 4device by a factor of about 100. An improvement in sensitivity by a factor of 1000 may be readily achieved and under some conditions the improvement is much greater.

Utilizing the apparatus in FIGURE l a trace, such as illustrated in FIGURES 2 and 3, appears on cathode ray oscilloscope 54, which trace is indicative of the amount of impurity present in -the gas admitted through the conduit 34. .Theinitial slope of the trace is directly proportional to the amount of impurity present. The peak value of the trace, as well as its average value, are also directly related to the amount of impurity present. The

This produces an increase inthe ions of the main gas.

apparatus can be calibrated empirically by admitting gas with known impurities into the chamber and observing the resultant trace on cathode ray oscilloscope54. The shape of the .trace Iis important, particularly where there are competing processes for the transfer of charge from Where a number of impurities are present, the magnet 20 can be adjusted to admit the ions of the various impurities succesively viewed on the oscilloscope. By comparing the resultant traces with traces produced using known gases, the unkown gases can be analyzed with much improved accuracy.-

FIGURE 2 shows typical oscilloscope traces of an analysis of helium which contained a neonimpurity v(as stated by the manufacturer) as no more than 50 parts per million. The principal ion during the period for which the discharge power was turned on was He+. Upon cessation of the discharge power, clustering converted some of the He+ into He2+ and the remainder was used in the charge transfer reaction He+{-Ne He-l-Net. The process He2++Ne 2He|Ne+ also apparently produces Net. Decay -of Ne+ and He2+ in the later times 'was due to electron-ion recombination and diffusion losses of ions to t-he walls of the source vessel.

FIGURE 3 shows typical oscilloscope traces observed -in a'gas mixture of N2r02 in the ratio of 100:1. The principal lions formed are N+ and N21, when the gas mixture is -bombarded with very high energy (20 meV) electrons. Upon cessation of the pulse two reactions occur to p indicate the presence of O2 in the mixture, namely 'Y Well known, so a simple measurement of the electron current provides knowledge of the absolute density of the generator 48.

In FIGURE 4 there is illustrated another form of the ion source 20 in which the output of the radio-frequency pulser 42 is applied vto rings 56 and 58 on the outside of the walls 32. In this case the walls 32 must not shield the gas from the radio-frequency signals applied to the rings 56 and 58. A D.C. voltage is applied to the pusher electrode 36 by the Ibattery 38 and serves the same purpose as inthe ion source 29 shown in FIGURE 1. As in the ion source shown in FIGURE 1, the energy supplied by the radio-frequency pulser ionizes the gas in chamber 30 in, pulses, and the rest of the apparatus operates in the same fashion as described above in connection with FIG- within the chamber 30. These high speed electrons may lbe produced in a conventional fashion b y a linear accelerator 62. The electrons are driven through the wall 60 in bursts under ,the control of pulses applied to the linear accelerator through amplifier 50 from the pulse Ionization of the gas by a pulse of rel-ativistic electrons have the following advantages over radiofrequency discharges:

(1) The specific ionization of relativistic electrons is ionization, which can be produced uniformly.

(2) Useful ionization densities (upto l part in l05 molecules) can be produced in short pulses (-106 sec.).

(3) Relativistic electrons are relatively inefficient in producing molecular excitations, so reactions in the afterglow experience a minimum perturbation from metastable molecular species.

(4) Reproducible measurements can be performed on infrequent single ionization pulses to minimize the buildup of reaction products in the sample gas.

(5) The ionized gas can be subjected to a known electricand magnetic-field configuration which need not be disturbed by the ionizing process, although the fields may be altered by subsequent charge motion.

' In addition to, or in lieu of, cathode ray oscilloscope vacuum tube voltmeter 64 and an integrating vacuum tube voltmeter 66. These voltmeters may be recording voltme-ters, the time constant of the meter 66 being such that the me-ter reads the average ion current.

Another readout device is illustrated in FIGURE 6. In this case the pulse from the pulse generator 48 is delayed in a conventional delay `circuit 68 and applied to control the opening of a normally closed gate circuit 70. The gate, when opened, -serves to pass the signal from cathode yfollower 52 to vacuum tube voltmeter '72, which may be a recording meter. yIn this case the pulse serves to open the gate for ya limited time at a predetermined interval following the rinitial pulse. This samples the output of cathode follower `52 and, hence, the ion current at a particular time following the ionizing pulse. Since the impurity ions are relatively negligible at the time of the ionizing pulse, the sampled signal is a measure of the rate of growth of the impurity ions and, hence `as explained above, a direct measure of the -amount of the impurity.

The slope may also be determined by making measurements of the ion current at two different times after the ionizing pulse and taking the ratio of the measurements. Each measurement could be made as described above in connection with the apparatus of FIGURE 6, using two delay circuits providing different delay times, two Igates and two meters. The ratio may be taken manually or by using vwell-known automatic devices.

Various other modifications can be made within the scope of this invention. The above description is illustrative of a preferred form of the invention; however, the invention is limited only by the following claims.

What is claimed is:

1. A method of detecting small impurities in samples of certain gases when the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said method `comprising applying an energy pulse to the gas to ionize a fraction of the gas, `discontinuing said pulse to permit the ytransfer of charge from the ions of t-he main part of the gas to the more easily ionized impurity so as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following the pulse, separating the ions according .to their mass, and selectively detecting separated ions of the mass of an impurity.

42. A method of detecting small impurities in samples of certain gases when the ionization potential of the irnpurity is less than the ionization potential of the main part of fthe gas, said method comprising applying an energy pulse to the gas to ionize a Ifraction of the gas, discontinuing said pulse to permit the yt-ransfer of charge from the ions of the main part of the -gas to the more easily ionized -impurity so .as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following the pulse, separating the -ions according to their mass, and selectively measuring the rate of increase of separated ions of the mass of an impurity in the lafterglow following cessation of the energy pulse.

3. A method of detecting small impurities in samples of certain gases when `the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said method comprising applying an energy pulse to the gas to ionize a ,fraction of the gas,

discontinuing [said pulse to permit the transfer of charge' from the ions of the ymain part of the gas to the more easily ionized impurity so as to increase the number of impurity -ions at the expense of the ions of rthe main part of -the gas during the interval following the pulse, separating the ions according to their m-ass, and selectively measuring the number of separated ions of the mass of an impurity 'at :a predetermined time in the afterglow following cessation of the energy pulse.

4. A method of detect-ing small Iimpurities in samples of certain gases when the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said method comprising periodically lapplying an energy pulse t-o the gas to ionize a fraction of the gas, discontinuing said pulses to permit the transfer of charge from the ions of the main part of the gas to the more easily ionized impurity so as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following the pulse, separating the ions according to ltheir mass, and selectively measuring the -average number of separated ions of the mass of an impurity.

5. A method of detecting small impurities in samples of certain gases when the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said method comprising applying an energy pulse to ythe gas to ionize la fraction of the gas, `discontinuing said pulse to permit Ithe transfer of charge from the ions of the :main part of the gas to `the more easily ionized impurity so as to incre-ase the number of impurity ions at the expense of the ions of the main part of the gas during .the interval following ythe pulse, separating the ions according to their mass, and selectively measuring the peak number of separated ions of the mass of an impurity in the afterglow following `cessation of the energy pulse.

6. A method of detecting small impurities in samples of certain gases when the ionization potential of the impurity is less than t-he ionization potenti-al of the impart of the gas, said method comprising periodically applying an energy pulse to the gas to ionize only a small fraction of the gas, discontinuing said pulses for an interval long -relative to the duration of the pulse to permit the transfer of charge from the ions of the main part of the gas to the more easily ionized impurity so as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the 4interval following the pulse, separating the ions in the gas according to their mass, selectively measuring the current of ions of the mass of the impurity in the afterglow following cessation of the ionizing pulses.

7. A method of detecting small impurities in certain gases when the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said method comprising introducing a sample of the gas into a chamber, applying pulses of electrical energy to the gas within the chamber to ionize said gas in pulses so that the charge from the ions of the main part of the gas is transferred to the more easily ion-ized impurity -to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following a pulse, withdrawing ions so produced from Ithe chamber, separating the ions leaving the chamber according to their mass, detecting the current of separated ions of a particular mass, and measuring the rate of increase of ion current thus detected following cessation of application of each pulse of electrical energy to the gas in said chamber.

8. A 4method of detecting small impurities in certain gases when the ionization potential of the impur-ity is less than the ionization potential of the main part of the gas, said Imethod comprising introducing the impure gas into a chamber, periodically applying pulses of electrical energy to the gas within the chamber to ionize said gas in pulses so ythat the charge from the ions of the main part of the gas is transferred to the more easily ionized impurity to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following a pulse, withdrawing ions so produced from the chamber, forming the ions leaving the chamber into a beam, separating the ions in lthe beam according to their mass, detecting the current of separated ions of a particular mass by producing a proportionally related electrical signal, applying the electrical signal so produced to electron Ibeam deflection me-ans of =a cathode ray oscilloscope having a time dependent sweep, and synchronizing said sweep with the ionizing pulses, whereby the time dependence of the impurity ion current is displayed upon the oscilloscope.

9. An ultra-sensitive mass spectrometer for detecting small impurities in certain gases when the ionization potential of the impurity is les than the ionization potential of lthe main part of the gas, said spectrometer comprising ian ion source, said ion source including ymeans for applying an energy pulse to the gas in the ion source to ionize a `fraction thereof and lfor discontinuing said pulse Vto permit the transfer of charge from the ions of the m-ain part of the gas to the more easily ionized impurity so .as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following the pulse; means for directing ions produced in the ion source into a beam; magnetic means for separating the ions ,in the ibeam according to their mass; and means for selectively measuring the peak cur-rent of separated ions of the mass of `an impurity following cessation of an energy pulse.

`10. An ultra-sensitive mass spectrometer for detecting small impurities in certain gases when the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said spectrometer comprising an ion source, said ion source including means .-for applying an energy pulse to the gas in the ion source to ionize a fraction -thereof and for discontinuing said pulse to permit the transfer of charge from the ions of the main part of the gas to the more easily ionized impurity so as to increase the number of impurity ions at the expense of the ions of the main part of the gas dur-ing the interval following the pulse, means for separating the ions produced in the ion source according to their mass; and means for selectively measuring the rate of increase of separated ions of the mass offan impurity in the afterglow following cessation of an energy pulse in the ion source.

1v1. An ultra-sensitive `mass spectrometer for detecting small impurities in ycertain gases When the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said spectrometer comprising an ion source, said ion source including walls denin-g a chamber, means for introducing the impure gas into said chamber, and means for applying 'a pulse of electrical energy to the gas Within said chamber thereby to ionize said gas and for discontinuing said pulse to permit -the transfer of charge lfrom the ions of the main part of the gas to the more easily ionized impurity so as to increase the number of impurity ions at the expense of the ions ofthe main part of the gas during the interval follow-ing the pulse, said walls being apertured to permit the ions thereby created to leave Isaid chamber; means for separating the ions leaving said chamber according to their mass; a detector of the current of separated ions of particular mass; and means for measuring the rate of increase of ion current detected by said detector following cessation of a pulse of electrical energy to the gas in said chamber.

12. An ultra-sensitive mass spectrometer for detecting small impurities -in certain gases when the ionization potential of the impurity is less than the ionization po- Itential of the main part of the gas, said spectrometer comprising an ion source, said ion source including walls defining a chamber, means for introducing the impure gas into said chamber, and means for applying a pulse of electrical energy to the gas within said chamber thereby to ionize said gas and for discontinuing said pulse to permit -the transfer of charge from the ions of 'the main part of the gas to the more easily ionized impurity so .as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following the pulse, said walls being apertured to permit the ions thereby created to leave said chamber; means for separating the ions leaving said chamber according to their mass; and means for selectively measuring the current of separated ions of the mass of an impurity yas a function of time following cessation of a pulse of electrical energy to the gas -in said cham-ber, said last mentioned means being synchronized swith said means for applying a pulse.

13. An ultra-sensitive mass spectrometer for detecting small impurities in certain gases when the ionization potential of the impurity is less than the ionization potential of the main part of the gas, said spectrometer comprising an ion source, said ion source including means for ionizing gas in the ion source; a pulse generator coupled to and controlling said means for ionizing gas, said pulse generator producing energy pulses short relative to the interval between pulses whereby the charge from the ions of the main part of the gas is transferred to themore easily ionized impurity so as to increase the number of impurity ions at the expense of the ions of the main part of the gas dur-ing the interval following a pulse; means for directing ions produced in the ion source into a beam; magnetic means for separating the ions in the beam according to their mass; means for selectively measuring t-he current of separated ions of the mass of an impurity; and synchronizing means connecting said pulse generator to said means `for measuring for synchronizing said means for measuring with the energy pulses.

14. An ultra-sesitive mass spectrometer for detecting small impurities in certain gases when the -ionization potential of the impurity is less than the ionization potential of the main part of the gas, said spectrometer comprising an ion source, said ion source including means for ionizing gas in the ion source; a :pulse generator coupled to and controlling said means yfor ionizing gas, said pulse generator periodically producing energy pulses short relative to the interval between pulses whereby the charge from the ions of the main part of the gas is transferred to the more easily ionized impurity so as to increase the number of impurity ions at the expense of the ions of the main part of the gas during the interval following a pulse;

means for directing ions produced in the ion source linto a beam; magnetic means for separating the ions in the beam according to their mass; a detector of the current of separated ions of particular mass; a cathode ray oscilloscope having electron beam deection means connected to receive the output of said detector and having Ia time dependent sweep circuit; and synchronizing means connecting said pulse generator to said sweep circuit for synchronizing the trace on said oscilloscope with the energy pulses.

References Cited bythe Examiner UNITED STATES PATENTS 9/1956 Hipple Z50-41.9

OTHER REFERENCES RALPH G. NILSON, Primm Examiner.l

H. S. MILLER, G. E. MATTHEWS, W.` F. LIND- QUIST, Assistant Examiners. 

1. A METHOD OF DETECTING SMALL IMPURITIES IN SAMPLES OF CERTAIN GASES WHEN THE IONIZATION POTENTIAL OF THE IMPURITY IS LESS THAN THE IONIZATION POTENTIAL OF THE MAIN PART OF THE GAS, SAID METHOD COMPRISING APPLYING AN ENERGY PULSE TO THE GAS TO IONIZE A FRACTION OF THE GAS, DISCONTINUING SAID PUSLE TO PERMIT THE TRANSFER OF CHARGE FROM THE IONS OF THE MAIN PART OF THE GAS TO THE MORE EASILY IONIZED IMPURITY SO AS TO INCREASE THE NUMBER OF IMPURITY IONS AT THE EXPENSE OF THE IONS OF THE MAIN PART OF THE GAS DURING THE INTERVAL FOLLOWING THE PULSE, SEPARATING THE IONS ACCORDING TO THEIR MASS, AND SELECTIVELY DETECTING SEPARATED IONS OF THE MASS OF AN IMPURITY.
 9. AN ULTRA-SENSITIVE MASS SPECTROMETER FOR DETECTING SMALL IMPURITIES IN CERTAIN GASES WHEN THE IONIZATION POTENTIAL OF THE IMPURITY IS LESS THAN THE IONIZATION POTENTIAL OF THE MAIN PART OF THE GAS, SAID SPECTROMETER COMPRISING AN ION SOURCE, SAID ION SOURCE INCLUDING MEANS FOR APPLYING AN ENERGY PULSE SO THE GAS IN THE ION SOURCE OT IONIZE A FRACTION THEREOF AND FOR DISCONTINUING SAID PULSE TO PERMIT THE TRANSFER OF CHARGE FROM THE IONS OF THE MAIN PART OF THE GAS TO THE MOVE EASILY IONIZED IMPURITY SO AS TO INCREASE THE NUMBER OF IMPURITY IONS AT THE EXPENSE OF THE IONS OF THE MAIN PART OF THE GAS DURING THE INTERVAL FOLLOWING THE PULSE; MEANS FOR DIRECTING IONS PRODUCED IN THE ION 